Polymers with low band gaps and high charge mobility

ABSTRACT

Polymers with low band gaps and high charge mobility, as well as related systems, methods and components are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Utility application Ser. No.11/485,708, filed Jul. 13, 2006, which in turn is a continuation-in-partof U.S. Utility application Ser. No. 11/450,521, filed Jun. 9, 2006,which in turn is a continuation-in-part of U.S. Utility application Ser.No. 11/375,643, filed Mar. 14, 2006, which claims priority to U.S.Provisional Application Ser. No. 60/699,123, filed Jul. 14, 2005. Thecontents of all parent applications are hereby incorporated byreference.

TECHNICAL FIELD

This disclosure generally relates to the field of electron donormaterials, as well as related photovoltaic cells.

BACKGROUND OF THE INVENTION

Photovoltaic cells are commonly used to transfer energy in the form oflight into energy in the form of electricity. A typical photovoltaiccell includes a photoactive material disposed between two electrodes.Generally, light passes through one or both of the electrodes tointeract with the photoactive material. As a result, the ability of oneor both of the electrodes to transmit light (e.g., light at one or morewavelengths absorbed by a photoactive material) can limit the overallefficiency of a photovoltaic cell. In many photovoltaic cells, a film ofsemiconductive material (e.g., indium tin oxide) is used to form theelectrode(s) through which light passes because, although thesemiconductive material can have a lower electrical conductivity thanelectrically conductive materials, the semiconductive material cantransmit more light than many electrically conductive materials.

SUMMARY

An aspect of the invention relates to a new combination of monomers thatproduce polymers, wherein the polymers have properties suitable for useas charge carriers in the active layer of a photovoltaic cell.

In one aspect, the invention features a class of co-polymers includingat least two co-monomers, at least one of which is acyclopentadithiophene.

In another aspect, this invention features a photovoltaic cell includinga first electrode, a second electrode, and a photoactive materialdisposed between the first and second electrodes. The photoactivematerial includes a polymer having a first comonomer repeat unit and asecond comonomer repeat unit. The first comonomer repeat unit includes acyclopentadithiophene moiety. The second comonomer repeat unit includesa silole moiety, a thienothiophene moiety, a thienothiophene oxidemoiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, ora tetrahydroisoindole moiety.

In another aspect, this invention features a photovoltaic cell includinga first electrode, a second electrode, and a photoactive materialdisposed between the first and second electrodes. The photoactivematerial includes a polymer having a first comonomer repeat unit and asecond comonomer repeat unit different from the first comonomer repeatunit. The first comonomer repeat unit includes a cyclopentadithiophenemoiety.

In another aspect, this invention features a polymer that includes afirst comonomer repeat unit containing a cyclopentadithiophene moiety,and a second comonomer repeat unit containing a benzothiadiazole moiety,a thiadiazoloquinoxaline moiety, a cyclopentadithiophene oxide moiety, abenzoisothiazole moiety, a benzothiazole moiety, a thiophene oxidemoiety, a fluorene moiety, a thiophene moiety, a silole moiety, athienothiophene moiety, a thienothiophene oxide moiety, adithienothiophene moiety, a dithienothiophene oxide moiety, atetrahydroisoindole moiety, or a moiety containing at least threethiophene moieties.

In another aspect, this invention features a polymer that includes afirst comonomer repeat unit and a second comonomer repeat unit differentfrom the first comonomer repeat unit. The first comonomer repeat unitcontains a cyclopentadithiophene moiety substituted with at least onesubstituent selected from the group consisting of hexyl, ethylhexyl,dimethyloctyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, andC₃-C₂₀ heterocycloalkyl.

In another aspect, this invention features a device (e.g., aphotovoltaic cell) that includes a first electrode, a second electrode,and a photoactive material disposed between the first and secondelectrodes. The photoactive material includes a polymer having a firstmonomer repeat unit, which includes a benzothiadiazole moiety, athiophene oxide moiety, a cyclopentadithiophene oxide moiety, athiadiazoloquinoxaline moiety, a benzoisothiazole moiety, abenzothiazole moiety, a thienothiophene moiety, a thienothiophene oxidemoiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, atetrahydroisoindole moiety, a fluorene moiety, a thiophene moiety, asilole moiety, or a fluorene moiety.

In another aspect, this invention features a device (e.g., aphotovoltaic cell) that includes a first electrode, a second electrode,and a photoactive material disposed between the first and secondelectrodes. The photoactive material includes a polymer having a firstmonomer repeat unit, which includes a cyclopentadithiophene moietysubstituted with at least one substituent selected from the groupconsisting of hexyl, ethylhexyl, dimethyloctyl, C₁-C₂₀ alkoxy, aryl,heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl halo, CN, NO₂, orSO₂R, in which R is C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl,C₃-C₂₀ cycloalkyl, or C₃-C₂₀ heterocycloalkyl.

Embodiments can include one or more of the following features.

In some embodiments, the cyclopentadithiophene moiety is substitutedwith at least one substituent selected from the group consisting ofC₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀heterocycloalkyl, halo, CN, NO₂, and SO₂R, in which R is H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl. Examples of C₁-C₂₀ alkyl can be hexyl, 2-ethylhexyl,or 3,7-dimethyloctyl.

In some embodiments, the cyclopentadithiophene moiety can be substitutedat 4-position.

In some embodiments, the first monomer or comonomer repeat unit caninclude a cyclopentadithiophene moiety of formula (I):

In formula (I), each of R₁, R₂, R₃, and R₄, independently, is H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀heterocycloalkyl, halo, CN, NO₂, or SO₂R, in which R is H, C₁-C₂₀ alkyl,C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl. In some embodiments, at least one of R₁ and R₂,independently, is hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl. In certainembodiments, each of R₁ and R₂, independently, is hexyl, 2-ethylhexyl,or 3,7-dimethyloctyl. In some embodiments, one of R₁ and R₂ is hexyl,ethylhexyl, dimethyloctyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀cycloalkyl, or C₃-C₂₀ heterocycloalkyl, the other of R₁ and R₂ is H,C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, orC₃-C₂₀ heterocycloalkyl. In some embodiments, at least one of R₁ and R₂,independently, is C₁-C₂₀ alkoxy optionally further substituted withC₁-C₂₀ alkoxy or halo (e.g., (OCH₂CH₂)₂OCH₃ or OCH₂CF₂OCF₂CF₂OCF₃). Incertain embodiments, each of R₁ and R₂, independently, is C₁-C₂₀ alkoxyoptionally further substituted with C₁-C₂₀ alkoxy or halo.

In some embodiments, the second comonomer repeat unit can include abenzothiadiazole moiety, a thiadiazoloquinoxaline moiety, acyclopentadithiophene oxide moiety, a benzoisothiazole moiety, abenzothiazole moiety, a thiophene oxide moiety, a thienothiophenemoiety, a thienothiophene oxide moiety, a dithienothiophene moiety, adithienothiophene oxide moiety, a tetrahydroisoindole moiety, a fluorenemoiety, a thiophene moiety, or a silole moiety, each of which isoptionally substituted with at least one substituent selected from thegroup consisting of C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl,C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂, and SO₂R, inwhich R is H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀cycloalkyl, or C₃-C₂₀ heterocycloalkyl. In some embodiments, the secondcomonomer repeat unit can include a 3,4-benzo-1,2,5-thiadiazole moiety.

In some embodiments, the second comonomer repeat unit can include abenzothiadiazole moiety of formula (II), a thiadiazoloquinoxaline moietyof formula (III), a cyclopentadithiophene dioxide moiety of formula(IV), a cyclopentadithiophene monoxide moiety of formula (V), abenzoisothiazole moiety of formula (VI), a benzothiazole moiety offormula (VII), a thiophene dioxide moiety of formula (VIII), acyclopentadithiophene dioxide moiety of formula (IX), or acyclopentadithiophene tetraoxide moiety of formula (X):

in which each of R₅, R₆, and R₇, independently, is H, C₁-C₂₀ alkyl,C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀heterocycloalkyl, halo, CN, NO₂, and SO₂R, in which R is H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl. In some embodiments, the second comonomer repeat unitcan include a benzothiadiazole moiety of formula (II). In certainembodiments, R₅ and R₆ is H.

In some embodiments, the second comonomer repeat unit can include atleast three thiophene moieties. In some embodiments, at least one of thethiophene moieties is substituted with at least one substituent selectedfrom the group consisting of C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl,heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂,and SO₂R, in which R is H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl,heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀ heterocycloalkyl. In certainembodiments, the second comonomer repeat unit includes five thiophenemoieties.

In some embodiments, the second comonomer repeat unit can include athienothiophene moiety of formula (XI), a thienothiophene tetraoxidemoiety of formula (XII), a dithienothiophene moiety of formula (XIII), adithienothiophene dioxide moiety of formula (XIV), a dithienothiophenetetraoxide moiety of formula (XV), a tetrahydroisoindole moiety offormula (XVI), a thienothiophene dioxide moiety of formula (XVII), or adithienothiophene dioxide moiety of formula (XVIII):

in which each of X and Y, independently, is CH₂, O, or S; each of R₅ andR₆, independently, is H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl,C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂, or SO₂R, inwhich R is C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀cycloalkyl, or C₃-C₂₀ heterocycloalkyl; and R₇ is H, C₁-C₂₀ alkyl,C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl.

In some embodiments, the polymer can further include a third comonomerrepeat unit that contains a thiophene moiety or a fluorene moiety. Insome embodiments, the thiophene or fluorene moiety is substituted withat least one substituent selected from the group consisting of C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, and C₃-C₂₀heterocycloalkyl, halo, CN, NO₂, and SO₂R, in which R is H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl.

In some embodiments, the first monomer or comonomer repeat unit caninclude a benzothiadiazole moiety of formula (II), a thiophene dioxidemoiety of formula (VIII), a cyclopentadithiophene tetraoxide moiety offormula (X), or a fluorene moiety of formula (XIX):

in which each of R₅ and R₆, independently, is H, C₁-C₂₀ alkyl, C₁-C₂₀alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl,halo, CN, NO₂, or SO₂R. R can be C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl,heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀ heterocycloalkyl. In someembodiments, at least one of R₅ and R₆ can be C₁-C₂₀ alkoxy optionallyfurther substituted with C₁-C₂₀ alkoxy or halo (e.g., (OCH₂CH₂)₂OCH₃ orOCH₂CF₂OCF₂CF₂OCF₃).

In some embodiments, the polymer can include a second monomer repeatunit different from the first monomer repeat unit. The second monomerrepeat unit can include a cyclopentadithiophene moiety, abenzothiadiazole moiety, a thiophene oxide moiety, acyclopentadithiophene oxide moiety, a fluorene moiety, or a thiophenemoiety.

In some embodiments, the first or second monomer repeat unit can includeat least one substituent on a ring selected from the group consisting ofC₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, C₃-C₂₀heterocycloalkyl, halo, CN, NO₂, and SO₂R, in which R is H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl. The substituent can be hexyl, ethylhexyl, or C₁-C₂₀alkoxy optionally further substituted with C₁-C₂₀ alkoxy or halo (e.g.,(OCH₂CH₂)₂OCH₃ or OCH₂CF₂OCF₂CF₂OCF₃).

In some embodiments, the second monomer repeat unit can include acyclopentadithiophene moiety of formula (I), a benzothiadiazole moietyof formula (II), a thiophene dioxide moiety of formula (VIII), acyclopentadithiophene tetraoxide moiety of formula (X), a fluorenemoiety of formula (XIX), a thiophene moiety of formula (XX), or a silolemoiety of formula (XXI):

in which each of R₁, R₂, R₃, R₄, R₅, R₆, R₇, and R₈, independently, isH, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl,C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂, or SO₂R. R can be C₁-C₂₀ alkyl,C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀heterocycloalkyl. In some embodiments, at least one of R₁, R₂, R₃, R₄,R₅, R₆, R₇, and R₈, can be C₁-C₂₀ alkoxy optionally further substitutedwith C₁-C₂₀ alkoxy or halo (e.g., (OCH₂CH₂)₂OCH₃ or OCH₂CF₂OCF₂CF₂OCF₃).

In some embodiments, when the second comonomer contains a silole moietyof formula (XXI), at least one of R₅, R₆, R₇ and R₈ can be C₁-C₂₀ alkyloptionally substituted with halo, or aryl optionally substituted withC₁-C₂₀ alkyl. In certain embodiments, each of R₅ and R₆, independentlycan be aryl optionally substituted with C₁-C₂₀ alkyl, and each of R₇ andR₈, independently, can be C₁-C₂₀ alkyl optionally substituted with halo.An example of a silole moiety is

In some embodiments, the polymer can be an electron donor material or anelectron acceptor material.

In some embodiments, the polymer can be

in which n can be an integer greater than 1.

In some embodiments, the photovoltaic cell can be a tandem photovoltaiccell.

In some embodiments, the photoactive material can include an electronacceptor material. In some embodiments, the electron acceptor materialcan be a fullerene (e.g., C61-phenyl-butyric acid methyl ester, PCBM).

In some embodiments, the polymer and the electron acceptor material eachcan have a LUMO energy level. The LUMO energy level of the polymer canbe at least about 0.2 eV (e.g., at least about 0.3 eV) less negativethan the LUMO energy level of the electron acceptor material.

In some embodiments, the device can be an organic semiconductive device.In certain embodiments, the device can be a member selected from thegroup consisting of field effect transistors, photodetectors,photovoltaic detectors, imaging devices, light emitting diodes, lasingdevices, conversion layers, amplifiers and emitters, storage elements,and electrochromic devices.

Embodiments can provide one or more of the following advantages.

In some embodiments, using a polymer containing a cyclopentadithiophenemoiety can be advantageous because the cyclopentadithiophene moiety cancontribute to a shift in the maximum absorption wavelength toward thered or near IR region of the electromagnetic spectrum. When such apolymer is incorporated into a photovoltaic cell, the current andefficiency of the cell can increase.

In some embodiments, substituted fullerenes or polymers containingsubstituted monomer repeat units (e.g., substituted with long-chainalkoxy groups such as oligomeric ethylene oxides or fluorinated alkoxygroups) can have improved solubility in organic solvents and can form anphotoactive layer with improved morphology.

In some embodiments, a polymer containing a silole moiety can absorblight at a relatively long wavelength and have improved solubility inorganic solvents. In some embodiments, a polymer containing a silolemoiety can be used to prepare an electron donor material with improvedsemiconductive properties.

In some embodiments, a polymer fullerene cell containing a polymerdescribed above can have a band gap that is relatively ideal for itsintended purposes.

In some embodiments, a photovoltaic cell having high cell voltage can becreated, whereby the HOMO level of the polymer is at least about 0.2electron volts more negative relative to the LUMO or conduction band ofan electron acceptor material.

In some embodiments, a photovoltaic cell containing a polymer describedabove can have relatively fast and efficient transfer of an electron toan electron acceptor material, whereby the LUMO of the donor is at leastabout 0.2 electron volt (e.g., at least about 0.3 electron volt) lessnegative than the conduction band of the electron acceptor material.

In some embodiments, a photovoltaic cell containing a polymer describedabove can have relatively fast charge separation, whereby the chargemobility of the positive charge, or hole, is relatively high and fallswithin the range of 10⁻⁴ to 10⁻¹ cm²/Vs.

In some embodiments, the polymer is soluble in an organic solvent and/orfilm forming.

In some embodiments, the polymer is optically non-scattering.

In some embodiments, the polymer can be used in organic field effecttransistors and OLEDs.

Other features and advantages of the invention will be apparent from thedescription, drawings, and claims.

DESCRIPTION OF DRAWING

FIG. 1 is a cross-sectional view of an embodiment of a photovoltaiccell.

FIG. 2 is a schematic of a system containing one electrode between twophotoactive layers.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional view of a photovoltaic cell 100 thatincludes a substrate 110, a cathode 120, a hole carrier layer 130, anactive layer 140 (containing an electron acceptor material and anelectron donor material), a hole blocking layer 150, an anode 160, and asubstrate 170.

In general, during use, light impinges on the surface of substrate 110,and passes through substrate 110, cathode 120, and hole carrier layer130. The light then interacts with active layer 140, causing electronsto be transferred from the electron donor material (e.g., a polymerdescribed above) to the electron acceptor material (e.g., PCBM). Theelectron acceptor material then transmits the electrons through holeblocking layer 150 to anode 160, and the electron donor materialtransfers holes through hole carrier layer 130 to cathode 120. Anode 160and cathode 120 are in electrical connection via an external load sothat electrons pass from anode 160, through the load, and to cathode120.

Electron acceptor materials of active layer 140 can include fullerenes.In some embodiments, active layer 140 can include one or moreunsubstituted fullerenes and/or one or more substituted fullerenes.Examples of unsubstituted fullerenes include C₆₀, C₇₀, C₇₆, C₇₈, C₈₂,C₈₄, and C₉₂. Examples of substituted fullerenes include PCBM orfullerenes substituted with C₁-C₂₀ alkoxy optionally further substitutedwith C₁-C₂₀ alkoxy or halo (e.g., (OCH₂CH₂)₂OCH₃ or OCH₂CF₂OCF₂CF₂OCF₃).Without wishing to be bound by theory, it is believed that fullerenessubstituted with long-chain alkoxy groups (e.g., oligomeric ethyleneoxides) or fluorinated alkoxy groups have improved solubility in organicsolvents and can form an photoactive layer with improved morphology.

In some embodiments, the electron acceptor materials can includepolymers (e.g., homopolymers or copolymers). A polymers mentioned hereininclude at least two identical or different monomer repeat units (e.g.,at least 5 monomer repeat units, at least 10 monomer repeat units, atleast 50 monomer repeat units, at least 100 monomer repeat units, or atleast 500 monomer repeat units). A copolymer mentioned herein refers toa polymer that includes at least two co-monomers of differingstructures. In some embodiments, the polymers used as an electronacceptor material can include one or more monomer repeat units listed inTables 1 and 2 below. Specifically, Table 1 lists examples of themonomers that can be used as an electron donating monomer and can serveas a conjugative link. Table 2 lists examples of the monomers that canbe used as an electron withdrawing monomer. Note that depending on thesubstituents, monomers listed in Table 1 can also be used as electronwithdrawing monomers and monomers listed in Table 2 can also be used aselectron donating monomers. Preferably, the polymers used as an electronacceptor material include a high molar percentage (e.g., at least about50%, at least about 60%, at least about 70%, at least about 80%, atleast about 90%) of an electron withdrawing monomer.

Electron donor materials of active layer 140 can include polymers (e.g.,homopolymers or copolymers). In some embodiments, the polymers used asan electron donor material can include one or more monomer repeat unitslisted Tables 1 and 2. Preferably, the polymers used as an electrondonor material include a high molar percentage (e.g., at least about50%, at least about 60%, at least about 70%, at least about 80%, atleast about 90%) of an electron donating monomer. In some embodiments,the polymers include a monomer containing C₁-C₂₀ alkoxy on a ring, whichis optionally further substituted with C₁-C₂₀ alkoxy or halo (e.g.,(OCH₂CH₂)₂OCH₃ or OCH₂CF₂OCF₂CF₂OCF₃). Without wishing to be bound bytheory, it is believed that polymers containing monomers substitutedwith long-chain alkoxy groups (e.g., oligomeric ethylene oxides) orfluorinated alkoxy groups have improved solubility in organic solventsand can form an photoactive layer with improved morphology.

TABLE 1

TABLE 2

Referring to formulas listed in Tables 1 and 2 above, each of X and Y,independently, can be CH₂, O, or S; each of R₁, R₂, R₃, R₄, R₅, R₆, R₇,and R₈, independently, can be H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl(e.g., phenyl or substituted phenyl), heteroaryl, C₃-C₂₀ cycloalkyl,C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂, or SO₂R; and R₇ can be H, C₁-C₂₀alkyl, C₁-C₂₀ alkoxy, aryl (e.g., phenyl or substituted phenyl),heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀ heterocycloalkyl; in which R isC₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, orC₃-C₂₀ heterocycloalkyl. An alkyl can be saturated or unsaturated andbranch or straight chained. A C₁-C₂₀ alkyl contains 1 to 20 carbon atoms(e.g., one, two, three, four, five, six, seven, eight, nine, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkylmoieties include —CH₃, —CH₂—, —CH₂═CH₂—, —CH₂—CH═CH₂, and branched—C₃H₇. An alkoxy can be branch or straight chained and saturated orunsaturated. An C₁-C₂₀ alkoxy contains an oxygen radical and 1 to 20carbon atoms (e.g., one, two, three, four, five, six, seven, eight,nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms).Examples of alkoxy moieties include —OCH₃ and —OCH═C₂H₄. A cycloalkylcan be either saturated or unsaturated. A C₃-C₂₀ cycloalkyl contains 3to 20 carbon atoms (e.g., three, four, five, six, seven, eight, nine,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examplesof cycloalkyl moieties include cyclohexyl and cyclohexen-3-yl. Aheterocycloalkyl can also be either saturated or unsaturated. A C₃-C₂₀heterocycloalkyl contains at least one ring heteroatom (e.g., O, N, andS) and 3 to 20 carbon atoms (e.g., three, four, five, six, seven, eight,nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms).Examples of heterocycloalkyl moieties include 4-tetrahydropyranyl and4-pyranyl. An aryl can contain one or more aromatic rings. Examples ofaryl moieties include phenyl, phenylene, naphthyl, naphthylene, pyrenyl,anthryl, and phenanthryl. A heteroaryl can contain one or more aromaticrings, at least one of which contains at least one ring heteroatom(e.g., O, N, and S). Examples of heteroaryl moieties include furyl,furylene, fluorenyl, pyrrolyl, thienyl, oxazolyl, imidazolyl, thiazolyl,pyridyl, pyrimidinyl, quinazolinyl, quinolyl, isoquinolyl, and indolyl.

Alkyl, alkoxy, cycloalkyl, heterocycloalkyl, aryl, and heteroarylmentioned herein include both substituted and unsubstituted moieties,unless specified otherwise. Examples of substituents on cycloalkyl,heterocycloalkyl, aryl, and heteroaryl include C₁-C₂₀ alkyl, C₃-C₂₀cycloalkyl, C₁-C₂₀ alkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy,amino, C₁-C₁₀ alkylamino, C₁-C₂₀ dialkylamino, arylamino, diarylamino,hydroxyl, halogen, thio, C₁-C₁₀ alkylthio, arylthio, C₁-C₁₀alkylsulfonyl, arylsulfonyl, cyano, nitro, acyl, acyloxy, carboxyl, andcarboxylic ester. Examples of substituents on alkyl include all of theabove-recited substituents except C₁-C₂₀ alkyl. Cycloalkyl,heterocycloalkyl, aryl, and heteroaryl also include fused groups.

The copolymers described above can be prepared by methods known in theart. For example, a copolymer can be prepared by a cross-couplingreaction between one or more comonomers containing two alkylstannylgroups and one or more comonomers containing two halo groups in thepresence of a transition metal catalyst. As another example, a copolymercan be prepared by a cross-coupling reaction between one or morecomonomers containing two borate groups and one or more comonomerscontaining two halo groups in the presence of a transition metalcatalyst. The comonomers can be prepared by the methods described hereinor by the methods know in the art, such as those described in Coppo etal., Macromolecules 2003, 36, 2705-2711 and Kurt et al., J. Heterocycl.Chem. 1970, 6, 629, the contents of which are hereby incorporated byreference.

Table 3 below lists three exemplary polymers (i.e., polymers 1-3)described in the Summary section above. These polymers can have uniqueproperties, which make them particularly suitable as charge carriers inthe active layer of a photovoltaic cell. Polymers 1 and 2 can beobtained by the methods described in Examples 4 and 7 below.

TABLE 3

Generally, one co-monomer in the polymers described in the Summarysection above is a cyclopentadithiophene. An advantage of a co-polymercontaining a cyclopentadithiophene moiety is that its absorptionwavelength can shift toward the red and near IR portion (e.g., 650-800nm) of the electromagnetic spectrum, which is not accessible by mostother polymers. When such a co-polymer is incorporated into aphotovoltaic cell, it enables the cell to absorb the light in thisregion of the spectrum, thereby increasing the current and efficiency ofthe cell.

The polymers described above can be useful in solar power technologybecause the band gap is close to ideal for a photovoltaic cell (e.g., apolymer-fullerene cell). The HOMO level of the polymers can bepositioned correctly relative to the LUMO of an electron acceptor (e.g.,PCBM) in a photovoltaic cell (e.g., a polymer-fullerene cell), allowingfor high cell voltage. The LUMO of the polymers can be positionedcorrectly relative to the conduction band of the electron acceptor in aphotovoltaic cell, thereby creating efficient transfer of an electron tothe electron acceptor. For example, using a polymer having a band gap ofabout 1.4-1.6 eV can significantly enhance cell voltage. Cellperformance, specifically efficiency, cam benefit from both an increasein photocurrent and an increase in cell voltage, and can approach andeven exceed 15% efficiency. The positive charge mobility of the polymerscan be relatively high and approximately in the range of 10⁻⁴ to 10⁻¹cm⁻²/Vs. In general, the relatively high positive charge mobility allowsfor relatively fast charge separation. The polymers can also be solublein an organic solvent and/or film forming. Further, the polymers can beoptically non-scattering.

Components in photovoltaic cell other than the electro acceptormaterials and the electron donor materials are known in the art, such asthose described in U.S. patent application Ser. No. 10/723,554, thecontents of which are incorporated herein by references.

In some embodiments, the polymer described above can be used as anelectron donor material or an electro acceptor material in a system inwhich two photovoltaic cells share a common electrode. Such a system isalso known as tandem photovoltaic cell. Examples of tandem photovoltaiccells are discussed in U.S. patent application Ser. No. 10/558,878,filed Nov. 29, 2005, the contents of which are hereby incorporated byreference.

As an example, FIG. 2 is a schematic of a tandem photovoltaic cell 200having a substrate 210, three electrodes 220, 240, and 260, and twophotoactive layers 230 and 250. Electrode 240 is shared betweenphotoactive layers 230 and 250, and is electrically connected withelectrodes 220 and 260. In general, electrodes 220, 240, and 260 can beformed of an electrically conductive material, such as those describedin U.S. patent application Ser. No. 10/723,554. In some embodiments, oneor more (i.e., one, two, or three) electrodes 220, 240, and 260 is amesh electrode. In some embodiments, one or more electrodes 220, 240,and 260 is formed of a semiconductive material. Examples ofsemiconductive materials include titanium oxides, indium tin oxides,fluorinated tin oxides, tin oxides, and zinc oxides. In certainembodiments, one or more (i.e., one, two, or three) electrodes 220, 240,and 260 are formed of titanium dioxide. Titanium dioxide used to preparean electrode can be in any suitable forms. For example, titanium dioxidecan be in the form of interconnected nanoparticles. Examples ofinterconnected titanium dioxide nanoparticles are described, forexample, in U.S. Pat. No. 7,022,910, the contents of which areincorporated herein by reference. In some embodiments, at least one(e.g., one, two, or three) of electrodes 220, 240, and 260 is atransparent electrode. As referred to herein, a transparent electrode isformed of a material which, at the thickness used in a photovoltaiccell, transmits at least about 60% (e.g., at least about 70%, at leastabout 75%, at least about 80%, at least about 85%, at least about 90%,at least about 95%) of incident light at a wavelength or a range ofwavelengths used during operation of the photovoltaic cell. In certainembodiments, both electrodes 220 and 260 are transparent electrodes.

Each of photoactive layers 230 and 250 can contain at least onesemiconductive material. In some embodiments, the semiconductivematerial in photoactive layer 230 has the same band gap as thesemiconductive material in photoactive layer 250. In certainembodiments, the semiconductive material in photoactive layer 230 has aband gap different from that of the semiconductive material inphotoactive layer 250. Without wishing to be bound by theory, it isbelieved that incident light not absorbed by one photoactive layer canbe absorbed by the other photoactive layer, thereby maximizing theabsorption of the incident light.

In some embodiments, at least one of photoactive layers 230 and 250 cancontain an electron acceptor material (e.g., PCBM or a polymer describedabove) and an electron donor material (e.g., a polymer described above).In general, suitable electron acceptor materials and electron donormaterials can be those described above. In certain embodiments, each ofphotoactive layers 230 and 250 contains an electron acceptor materialand an electron donor material.

Substrate 210 can be formed of one or more suitable polymers, such asthose described in U.S. patent application Ser. No. 10/723,554. In someembodiments, an additional substrate (not shown in FIG. 2) can bedisposed on electrode 260.

Photovoltaic cell 200 can further contain a hole carrier layer (notshown in FIG. 2) and a hole blocking layer (not shown in FIG. 2), suchas those described in U.S. patent application Ser. No. 10/723,554.

While photovoltaic cells have been described above, in some embodiments,the polymers described herein can be used in other devices and systems.For example, the polymers can be used in suitable organic semiconductivedevices, such as field effect transistors, photodetectors (e.g., IRdetectors), photovoltaic detectors, imaging devices (e.g., RGB imagingdevices for cameras or medical imaging systems), light emitting diodes(LEDs) (e.g., organic LEDs or IR or near IR LEDs), lasing devices,conversion layers (e.g., layers that convert visible emission into IRemission), amplifiers and emitters for telecommunication (e.g., dopantsfor fibers), storage elements (e.g., holographic storage elements), andelectrochromic devices (e.g., electrochromic displays).

The following examples are illustrative and not intended to be limiting.

Example 1 Synthesis of4,4-Dihexyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene

4H-Cyclopenta[2,1-b;3,4-b′]dithiophene was synthesized according toliterature procedure illustrated in Coppo et al., Macromolecules 2003,36, 2705-2711. All other starting materials were purchased fromSigma-Aldrich and used as received.

4H-Cyclopenta[2,1-b;3,4-b′]dithiophene (1.5 g, 0.00843 mol) wasdissolved in DMSO (50 mL). The solution was purged with nitrogen, andgrounded KOH (1.89 g, 0.0337 mol) and sodium iodide (50 mg) were added,followed by hexyl bromide (3.02 g, 0.0169 mol). The reaction was stirredfor 17 h under nitrogen at room temperature. Water was added and thereaction was extracted with t-butyl-methyl ether. The organic layer wasseparated and dried over magnesium sulfate. Solvent was removed undervacuum and the residue was purified by chromatography using hexanes aseluent. Fractions containing pure4,4-dixeyl-4H-cyclopenta[2,1-b;3,4-b]dithiophene product were combinedand the solvents evaporated. The product was obtained as a colorlessoil. Yield: 2.36 g (81%).

Example 2 The Synthesis of4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene

Starting material 4,4-dihexyl-4H-cyclopenta[2,1-b;3,4-b]dithiophene (1.5g, 0.00433 mol) was dissolved in dry THF (30 mL). The solution wascooled to −78° C. and butyl lithium (6.1 mL, 0.0130 mol) was added dropwise. The reaction was stirred at this temperature for 2 h and warmed toroom temperature, stirred for 3 h. Again reaction was cooled to −78° C.and trimethyltin chloride (1 M in hexanes, 16.0 mL, 16.0 mmol) was addeddropwise. The reaction was allowed to warm to rt and stirred for 17 h.Water was added and the reaction was extracted with toluene. The organiclayer was washed with water and dried over sodium sulfate. Solvent wasremoved under vacuum and the residue was dissolved in toluene, andquickly passed through a plug of silica gel pretreated with triethylamine. Solvent was removed and the residue dried under vacuum to afford2.65 g of the bis(trimethyltin) monomer. ¹H NMR (CDCl₃, 200 MHz): 6.97(m, 2H), 1.84 (m, 4H), 1.20 (m, 16H), 0.88 (m, 6H), 0.42 (m, 18H).

Example 3 The Synthesis ofbis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene

4,4-Dihexyl-4H-cyclopenta[2,1-b;3,4-b]dithiophene (2.2 g, 0.0065 mol)was dissolved in dry THF (20 mL). The solution was cooled to −78° C.BuLi (7.62, 2.5 M in hexanes, 0.019 mol) was then added to the solution.The reaction mixture was allowed to warm to room temperature and wasstirred for 5 hours. The mixture was then cooled again to −78° C. andBu₃SnCl (7.44 g, 0.0229 mol) was added. The reaction mixture was allowedto warm to room temperature and was stirred for another 48 hours. Waterwas then added and the mixture was extracted with dihicholomethane.Organic layer was collected, dried over anhydrous Na₂SO₄, andconcentrated. The residue thus obtained was dissolved in hexane andquickly passed through a plug of silica gel pretreated withtriethylamine. The solvent was removed and the residue was dried undervacuum to affordbis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene(5.7 g).

Example 4 Polymerization ofbis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiopheneand 4,7-dibromo-2,1,3-benzothiadiazole

Bis-(tributylstannyl)-4,4-dihexyl-cyclopenta[2,1-b:3,4-b′]dithiophene(0.775 g, 0.000816 mol) and 4,7-dibromo-2,1,3-benzothiadiazole (0.24 g,0.000816 mol) were first dissolved in toluene. After the reaction waspurged with nitrogen, palladium tretakistriphenylphosphine (15 mg,0.0065 mmol) was added. The reaction mixture was heated at 100° C. for24 hour. After the solvent was removed, the residue was washed withacetone and extracted in a Soxlet extractor for 8 hours to afford theproduct as an insoluble blue solid.

Example 5 Synthesis of4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene

4H-Cyclopenta[2,1-b;3,4-b]dithiophene (1.5 g, 0.00843 mol) was dissolvedin DMSO (50 mL). After the solution was purged with nitrogen, andgrounded KOH (1.89 g, 0.0337 mol), sodium iodide (50 mg), and2-ethylhexyl bromide (3.25 g, 0.0169 mol) were sequentially added. Thereaction mixture was stirred overnight under nitrogen (c.a. 16 hours).Water was added and the reaction was extracted with t-butylmethyl ether.The organic layer was collected, dried over magnesium sulfate, andconcentrated. The residue was purified by chromatography using hexanesas eluent. Fractions containing pure4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b]dithiophene productwere combined and concentrated. The product was obtained as a colorlessoil after drying under vacuum. Yield: 2.68 g (79%). ¹H NMR (CDCl₃, 250MHz): 7.13 (m, 2H), 6.94 (m, 2H), 1.88 (m, 4H), 0.94 (m, 16H), 0.78 (t,6.4 Hz, 6H), 0.61 (t, 7.3 Hz, 6H).

Example 6 Synthesis of4,4-Bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene

Starting material4,4-Bis-(2-ethyl-hexyl)-4H-cyclopenta[2,1-b;3,4-b]dithiophene (1.5 g,0.00372 mol) was dissolved in dry THF (20 mL). After the solution wascooled to −78° C., butyl lithium (5.21 mL, 0.0130 mol) was addeddropwise. The reaction mixture was stirred at this temperature for 1hour. It was then warmed to room temperature and stirred for another 3hours. The mixture was again cooled to −78° C. and trimethyltin chloride(1 M in hexane, 15.6 mL, 15.6 mmol) was added dropwise. The reactionmixture was allowed to warm to room temperature and stirred overnight(c.a. 16 hours).

Water was added and the reaction was extracted with toluene. The organiclayer was washed with water, dried over sodium sulfate, andconcentrated. The residue was dissolved in toluene, and quickly passedthrough a small plug of silica gel pretreated with triethylamine. Thesolvent was removed and the residue was dried under vacuum. 1.25 g ofthe product was obtained. ¹H NMR (CDCl₃, 250 MHz): 6.96 (m, 2H), 1.85(m, 4H), 1.29 (m, 2H), 0.92 (m, 16H), 0.78 (t, 6.8 Hz, 6H), 0.61 (t, 7.3Hz, 6H), 0.38 (m, 18H).

Example 7 Polymerization ofBis-(trimethylstannyl)-4,4-Di(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b′]dithiophenand 4,7-dibromo-2,1,3-benzothiadiazole

Bis-(trimethylstannyl)-4,4-di(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b]dithiophene(0.686 g, 0.000943 mol) and 4,7-dibromo-2,1,3-benzothiadiazole (0.269 g,0.000915 mol) were dissolved in toluene (20 mL). After the reaction waspurged with nitrogen, tris(dibenzylideneacetone)dipalladium(0) (25.1 mg,0.0275 mmol) and triphenylphosphine (57.6 mg, 0.220 mmol) were added.The reaction was further purged with nitrogen for 10 minutes and heatedto 120° C. under nitrogen for 24 hours. The solvent was removed undervacuum and the residue was dissolved in chloroform. After the mixturewas poured into methanol (500 mL), the blue precipitate thus obtainedwas collected by filtration, washed with methanol, and dried. Theprecipitate was dissolved in chloroform (30 mL) under heating, andfiltered through a 0.45 μm membrane. The solution was loaded on torecycling HPLC (2H+2.5H column on a Dychrome recycling HPLC, 5 cyclesfor each injection), in 3 mL portions for purification.Higher-molecular-weight fractions were combined to give 120 mg purepolymer (Mn=35 kDa).

Example 8 Copolymerization of4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene,4,4-Bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b′]dithiophene,and 4,7-Dibromo-benzo[1,2,5]thiadiazole

4,4-Dihexyl-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b]dithiophene(0.0863 g, 0.000128 mol),4,4-bis-(2-ethyl-hexyl)-2,6-bis-trimethylstannanyl-4H-cyclopenta[2,1-b;3,4-b]dithiophene(0.187 g, 0.000257 mol), and 4,7-Dibromo-benzo[1,2,5]thiadiazole (0.111g, 0.000378 g) were dissolved in toluene (15 mL) and the solution wasdegassed and purged with N₂. Tris(dibenzylideneacetone)dipalladium(0)(6.78 mg, 0.0074 mmol) and triphenylphosphine (15.5 mg, 0.0 593 mmol)were then added. The reaction was purged again with nitrogen for 30minutes and heated at 120° C. under nitrogen. The solvent was thenremoved under vacuum. The residue was dissolved in chloroform and thesolution was added into methanol. The precipitates were collected andextracted with hexane for 24 hours and then extracted with chloroformfor 8 hours. The resultant blue solution was concentrated and added tomethanol. The precipitates were collected to afford a first fraction ofthe polymer (70 mg). The remaining materials on the thimble was furtherextracted with chloroform for 20 hours. 20 mg additional polymer wascollected.

Example 9 Preparation of4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]thiophene-2,6-bis(pinacolborate)ester

100 mL oven dried Schlenk flask was charged with 1.097 g (2.72 mmol) of4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene. The flaskwas evacuated and purged with argon three times. To this flask was thenadded 20 mL of dry, distilled THF. The resulting solution was cooled to−78° C. and 4.35 mL (10.88 mmol, 4 equiv.) of 2.5M BuLi was addeddropwise. The reaction was stirred for 1 hour at −78° C. and then warmedto room temperature and stirred for an additional 3 hours. The solutionwas cooled again to −78° C. and 2.77 mL (13.6 mmol, 5 equiv.) of2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added in oneportion via syringe. The reaction was stirred at −78° C. for 1 hour andthen allowed to warm to room temperature overnight. The solution waspoured into water and extracted with 4×150 mL of methyl tert-butylether. The organic layers were combined and washed with 2×150 mL ofbrine, dried with anhydrous MgSO₄, and filtered. The solvent was removedunder vacuum to yield and orange oil, which was purified by columnchromatography (5% EtOAc in hexanes) to yield a colorless, viscous oil,1.34 g (75% yield).

Example 10 Preparation of a Pentathienyl-Cyclopentadithiophene Copolymer

A 50 mL Schlenk flask was charged with 0.309 g (0.472 mmol) of4H-4,4-bis(2′-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-bis(pinacolborate)ester prepared in Example 9, 0.367 g (0.510 mmol) of5,5′-dibromo-3″,4″-dihexyl-a-pentathiophene (its synthesis was describedin WO 2005/092947, which is incorporated herein by reference) 0.0013 g(0.00185 mmol) of PdCl₂(PPh₃)₂, and 0.057 g (0.142 mmol) oftrioctylmethylammonium chloride (Aliquot 336, Aldrich, St. Louis, Mo.).The flask was fitted with a reflux condenser and the flask was evacuatedand refilled with nitrogen three times. The solids were dissolved in 6mL of toluene and then 0.88 mL of 2M Na₂CO₃ were added via syringe. Thereaction was then heated to 95° C. with stiffing for 5 hours.Phenylboronic acid (0.031 g, 0.250 mmol) and 0.0016 g (0.00228 mmol) ofPdCl₂(PPh₃)₂ were dissolved in 1 mL of THF and added to the reactionmixture, and stiffing was continued for 16 h at 95° C. The reactionmixture was diluted with toluene (50 mL) and the organic layer wasseparated and washed with warm water (3×50 mL). The solution was thentreated with an aqueous solution of diethyldithiocarbamic acid sodiumsalt trihydrate (7.5%, DDC, 5 mL) and heated at 80° C. overnight. Theaqueous layer was separated and discarded and the organic layer waswashed with warm water (3×50 mL) and the polymer precipitated intomethanol (500 mL). The polymer was collected by filtration, washed withmethanol (50 mL) and redissolved in hot toluene (200 mL). The hotpolymer solution was passed through a tightly packed column of celite(1×8 cm), silica get (3×8 cm), and basic alumina (3×8 cm) (previouslyrinsed with 200 mL of hot toluene). The polymer solution was collectedand the volume concentrated to approximately 50 mL. The polymer wasprecipitated into methanol (500 mL), washed with methanol (100 mL),acetone (100 mL) and again with methanol (100 mL). The polymer was thendried in vacuo overnight to yield a brick red material. Yield: 0.327 g.

Example 11 Fabrication of Solar Cell

The polymer solar cells were fabricated by doctor-blading a blend of thepolymer prepared in Example 7 (PCPDTBT) and PC₆₁BM or PC₇₁BM (purchasedfrom Nano-C, Westwood, Mass.) in a 1:3 w/w ratio sandwiched between atransparent anode and an evaporated metal cathode. The transparent anodewas an indium tin oxide (ITO)-covered glass substrate (Merck, WhitehouseStation, N.J.) which was coated with a ˜60 nm thick PEDOT:PSS layer(Baytron PH from H. C. Starck) applied by doctorblading. TheITO-glass-substrate was cleaned by ultrasonification subsequently inacetone, isopropyl alcohol and deionized water. The cathode, a bilayerof a thin (1 nm) LiF layer covered with 80 nm Al, was prepared bythermal evaporation. PCPDTBT and PC₆₁BM or PC₇₁BM were dissolvedtogether in o-dichlorobenzene (ODCB) to give an overall 40 mg/mlsolution and was stirred overnight at 60-70° C. inside a glovebox. Theactive layer thickness, as determined by AFM, was between 150-250 nm.Device characterization was done under AM 1.5G irradiation (100 mW/cm²)on an Oriel Xenon solar simulator with a well calibrated spectralmismatch of 0.98 jV-characteristics were recorded with a Keithley 2400.Active areas were in the range of 15 to 20 mm². EQE was detected with alock-in amplifier under monochromatic illumination. Calibration of theincident light was done with a monocrystalline silicon diode. Mobilitymeasurements were done using an Agilent 4155C parameter analyzer.Absorption measurements were done inside the glovebox with an Avantesfiberoptic spectrometer or outside with a HP spectrometer.

The interaction with PCBM and the photoinduced charge transfer wasinvestigated by PL quenching. The PL of pristine PCPDTBT versusPCPDTBT/PCBM composites was measured at liquid N₂ temperatures in acryostat, excitation was provided by an Ar laser at 488 nm.

Electrochemical experiments were carried out on dropcast polymer filmsat room temperature in a glovebox. The supporting electrolyte wastetrabutylammonium-hexafluorophosphate (TBAPF₆, electrochemical grade,Aldrich) ˜0.1 M in acetonitrile anhydrous (Aldrich). The workingelectrode (WE), as well as the counter electrode (CE), was a platinumfoil. A silver wire coated with AgCl was used as a reference electrode(RE). After each measurement, the RE was calibrated with ferrocene(E⁰=400 mV vs. NHE) and the potential axis was corrected to NHE (using−4.75 eV for NHE^(24,25)) according to the difference of E⁰ (ferrocene)and the measured E^(1/2) (ferrocene). λ_(max) (CHCl₃)=710 nm,λ_(band edge) (CHCl₃)=780 nm, band gap (CHCl₃)=1.59 eV, λ_(max)(film)=700-760 nm, λ_(band edge) (film)=855 nm, band gap (film)=1.45 eV,HOMO=−5.3 eV, −5.7 eV (electrochem), LUMO=−3.85 eV, −4.25 eV, μ₊=2×10⁻²cm₂/Vs (TOF), 1×10⁻³ cm²/Vs (FET).

Other embodiments are in the claims.

1. A photovoltaic cell, comprising: a first electrode, a secondelectrode, and a photoactive material disposed between the first andsecond electrodes, the photoactive material comprising an electron donormaterial and an electron acceptor material, the electron donor materialcomprises a copolymer including a first monomer repeat unit, and thefirst monomer repeat unit comprises a thienothiophene moiety.
 2. Thephotovoltaic cell of claim 1, wherein the thienothiophene moiety isoptionally substituted with at least one substituent selected from thegroup consisting of C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl,C₃-C₂₀ cycloalkyl, C₃-C₂₀ heterocycloalkyl, halo, CN, NO₂, and SO₂R, inwhich R is H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀cycloalkyl, or C₃-C₂₀ heterocycloalkyl.
 3. The photovoltaic cell ofclaim 1, wherein the thienothiophene moiety is optionally substitutedwith halo.
 4. The photovoltaic cell of claim 1, wherein thethienothiophene moiety is optionally substituted with fluoro.
 5. Thephotovoltaic cell of claim 1, wherein the electron acceptor materialcomprises a fullerene.
 6. The photovoltaic cell of claim 1, wherein theelectron acceptor material comprises a substituted fullerene.
 7. Thephotovoltaic cell of claim 1, wherein the electron acceptor materialcomprises a PCBM.